linux-kernel - Re: [PATCH] sched/rt: Fix rq's cpupri leak while enqueue/dequeue child RT entities

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Message-ID: <20131213154211.GP21999@twins.programming.kicks-ass.net>
Date:	Fri, 13 Dec 2013 16:42:11 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Steven Rostedt <rostedt@...dmis.org>, stable@...r.kernel.org
Subject: Re: [PATCH] sched/rt: Fix rq's cpupri leak while enqueue/dequeue
 child RT entities

On Wed, Nov 27, 2013 at 07:59:13PM +0400, Kirill Tkhai wrote:
> This patch touches RT group scheduling case.
> 
> Functions inc_rt_prio_smp() and dec_rt_prio_smp() change (global) rq's priority,
> while rt_rq passed to them may be not the top-level rt_rq. This is wrong, because
> changing of priority on a child level does not guarantee that the priority is
> the highest all over the rq. So, this leak makes RT balancing unusable.
> 
> The short example: the task having the highest priority among all rq's RT tasks
> (no one other task has the same priority) are waking on a throttle rt_rq.
> The rq's cpupri is set to the task's priority equivalent, but real
> rq->rt.highest_prio.curr is less.
> 
> The patch below fixes the problem.
> 
> It looks like all version have this bug, so I CC'ed stable mailing list.

Yeah, I think this is right.

cpupri stuff should indeed only be changed for the top level group.
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